It is well known to deposit successive layers of optionally-doped silica inside a tube of fluorine-containing silica by injecting into the tube a gaseous mixture of silicon halide, oxygen, and a small quantity of a compound of a doping element that increase or decrease the refractive index of the silica, and then heating this mixture, either from the outside of the tube by means of a moving torch, or else by forming a moving plasma.
Nevertheless, this method suffers from certain shortcomings, since in practice the quantity of silica deposited is limited, with the area of the right cross-section of the deposited doped silica hardly being capable of exceeding 150 mm.sup.2, and with the ratio of the diameter of the optical cladding obtained after drawing to the diameter of the core remaining greater than 7. This means that no more than 20 km of optical fiber can be drawn from one meter of preform.
An object of the present invention is to provide a method of fabricating such preforms which enables larger quantities of silica to be deposited, with higher ratios of optical cladding diameter to core diameter, and thereby enabling greater lengths of optical fiber to be obtained per unit length of preform, it being possible to reach 100 km to 200 km of optical fiber per meter of preform.